Probably sith leader, like Palpatine, so he heals when he inflicts Pain and better potency. Also having Nihilus on your squad will help reduce the cooldown of Sion's best ability. I don't really know from experience, because I went for Visas Marr instead of Sion.

My guess there are several teams he'd work in.
EP lead is the current fashion, and he works there. He works as a taunt in that team, and the fact he constantly debuffs enemies will add a lot to his survivability.
Nihilus lead would also work, since they have some synergy as a fellow Triumvirate member. Plus, Sion with that much health steal, with his damage, and a "revive with 100% health" ability when his protection gets converted into health... Could be deadly.
Maul lead could work as well if you've got him ready, or looking to fill in your TW/TB team. With all the R2s running around in arena, that might not be the best option, but it's there.
Traya lead will take time. Not even the biggest krakens will get her for a few months. If she becomes the meta, Sion will have a place by her side.

To beat the “Nightmare” team (EP, Vader, Sith Trooper, DN & Sion) I intend to run Rex, GK, Old Ben, DN & Sion.

The next few months will be a temporary meta until Traya takes hold. Traya, DN, Sion +2.

why old ben instead of shore trooper/sith trooper or a different taunting/pretaunting tank? if running cls lead, OB makes sense b/c if he gets resisted, your team will get a lot of TM but unless you get tenacity up first (with a rex etc), DN delays everybody's ability

My old ben and sion are only gear 9 so it is just a theorycrafted team. Idea being 3 durable tanks to protect Nihilus. Maybe another tank would be better in old ben’s place but mind tricks is no joke, can really disrupt the enemy if he goes quick.

Unfortunately i do not have Darth Nihilus or Sith trooper, so can't use the nightmare sith team. I currently run zmaul, zavage, EP, Vader, Sidious and also have a six star Sith Assassin. Using those characters what would be a good setup with Sion? I hear that Sion isn't very good under zmaul leadership so was thinking EP lead, with Sion, Zavage, Maul, Vader

Unfortunately i do not have Darth Nihilus or Sith trooper, so can't use the nightmare sith team. I currently run zmaul, zavage, EP, Vader, Sidious and also have a six star Sith Assassin. Using those characters what would be a good setup with Sion? I hear that Sion isn't very good under zmaul leadership so was thinking EP lead, with Sion, Zavage, Maul, Vader

zMaul and Zavage are no use with Sion. Best and only lead is EP rn since nobody will have Traya. DN is a must, ST is good, but Shore will replace him. Sidious and SA: basically all other Sith, excluding Traya are useless. So farm ST and DN. This is also why I am farming ST for Nightmare. Lol

Unfortunately i do not have Darth Nihilus or Sith trooper, so can't use the nightmare sith team. I currently run zmaul, zavage, EP, Vader, Sidious and also have a six star Sith Assassin. Using those characters what would be a good setup with Sion? I hear that Sion isn't very good under zmaul leadership so was thinking EP lead, with Sion, Zavage, Maul, Vader

zMaul and Zavage are no use with Sion. Best and only lead is EP rn since nobody will have Traya. DN is a must, ST is good, but Shore will replace him. Sidious and SA: basically all other Sith, excluding Traya are useless. So farm ST and DN. This is also why I am farming ST for Nightmare. Lol

ST is Shore Trooper or Sith Trooper? not good using that since they both have same initials...

Unfortunately i do not have Darth Nihilus or Sith trooper, so can't use the nightmare sith team. I currently run zmaul, zavage, EP, Vader, Sidious and also have a six star Sith Assassin. Using those characters what would be a good setup with Sion? I hear that Sion isn't very good under zmaul leadership so was thinking EP lead, with Sion, Zavage, Maul, Vader

zMaul and Zavage are no use with Sion. Best and only lead is EP rn since nobody will have Traya. DN is a must, ST is good, but Shore will replace him. Sidious and SA: basically all other Sith, excluding Traya are useless. So farm ST and DN. This is also why I am farming ST for Nightmare. Lol

ST is Shore Trooper or Sith Trooper? not good using that since they both have same initials...

I'm currently running Palp, Vader, Sion, sith Trooper, and Nihilus. Everyone is fully zeta'd, and I need a few pieces to get Trooper and vader to g12 (Sion is 4* g11, but he still functions well). Got to #11 in Arena this past day, the highest I've ever gotten in my launch shard. I'm having a lot of fun with the team. Funny about that video posted above, I have beaten several Rex teams with Wampa, Talzin, and the like.

I would say there is a learning curve with this new Sith team, but it is absolutely competitive. It can beat any other team out there. Once I unlock Traya, I expect the team to be even more dominant.